The effects of metal ions on nucleic acids structure and function have been determined. The primary focus now is on the mechanism of RNA synthesis at the active site of RNA polymerase. The geometry of interaction at the active site of the enzyme is being probed during the transcription process to determine how this geometry changes during transcription and affects its mechanism. Evidence has been obtained that the enzyme assures fidelity in copying the genetic code by conformation changes sensitive to complementarity or non-complementarity between DNA base and the base on the incoming nucleoside triphosphate. A comprehensive mechanism of fidelity has been developed that allows a Mg (II) switch to effect or prevent incorporation of the right and wrong substrate, respectively.